I personally think you site has been ready to turn to paid since the last PR update.
Whatever you price, you will be suprised that diserning people will still buy from you. One day, I put Haabaa.com permanent inclusion to £90 (about $180) I was not expecting to have any customers any more but suprise surprise I still have lots of customer and two of them even have and account with me because they add sites continously.
And when Haabaa dropped to PR5, I though the submission will dry up but I really have not noticed significant drop in submission compared with when it was PR6.
Decide you price and go for it Bagi, you have nothing to lose. I think like me your directory is not your main income sources so you can experiment
